The present invention relates to a hand circular saw and in particular to a plunge saw.
Hand circular saws are generally known in the art. One of such hand circular saws is disclosed in the German document DE-OS 3,912,307. In this saw the turning point for depth adjustment is located on the base plate, in particular on its region which faces the operator, and the saw blade is turnable outwardly of a stationary housing. Moreover, the spring means acts on the plunge saw against the base plate so that it has a tendency to maintain the upper dead point position or in other words the plunge depth zero and is releasably arrested by a mechanism in the zero position.
A standard hand circular saw is also disclosed in the German document DE-OS 1,503,927. In this saw the height and inclination adjustment of the saw blade is supported in a joint spherical hinge.
All known circular saws are designed so that the saw teeth during rotation of the saw blade press against the working table during engagement into the workpiece. For hand circular saw this means that the saw teeth of the saw blade which raise with respect to workpiece or in other words in the feeding direction must provide cutting. Therefore, under the action of the cutting force the workpiece is pulled against the ground plate of the saw. The danger of sudden springing and lifting off the saw from the workpiece due to the saw teeth which cut in opposite direction is almost prevented. This action of the saw teeth is identified as a counter run. The counter run is accepted by experts for safety reasons and it is considered as a standard for all known circular saws. The opposite case or straight run is very dangerous. It must be basically avoided. For this reason, for example a reverse of the displacement of the hand circular saws against the operators is prohibited.
In order to facilitate the understanding of the following considerations and to understand the present invention the following explanations are provided hereinbelow. The feeding direction is defined as the direction from the left to the right. The geometrical observation of the hand circular saws is performed from the right side as considered in the feeding direction. The saw blade rotates to the left. The center point of the saw blade is located always, more or less, above the turning point for lifting and lowering of the saw blade. The turning point for the plunging saws is located at the left before the center point, and in standard hand circular saws at the right before the center point of the saw blade. During turning up and down the saw blade is guided over a circular path. Thereby during immersing a vertical and a horizontal feed component is produced. The horizontal feed component leads to a predominant engagement of the saw teeth in the horizontal feed direction.
For providing a counter run during immersion, from the geometrical observation, presuming a uniform rotary direction of the saw blade, the hinge for lifting and lowering the saw blade must be located at the left, or in other words facing toward the operator.
Due to the above described conditions during plunge sawing the counter run takes place with the cut performed both over the workpiece and also vertically into the workpiece. It has been considered a technical advantage with respect to the standard hand circular saws. A substantial feature of the plunge saws is therefore that in immovable position with the cutting depth "zero" the saw teeth are positioned at a substantial distance above the base plate. Thus, the saw can be placed on a workpiece reliably and flush with the base plate.
The opposite is true for the standard hand circular saws. The straight run is produced here due to the horizontal feed component directed against the feed direction during immersion of the saw blade while predominantly the saw teeth lowering in the workpiece produce cutting.
Due to the geometry of the standard hand circular saw the saw teeth during immersion of the saw blade into a workpiece operate predominantly in the straight run, this saw case must be excluded for standard hand circular saws. Therefore they are designed so that in the immovable position with the cutting depth zero the saw teeth of the saw blade facing the base plate are not only positioned at no distance above the base plate but also in many cases have an extension projecting through the base plate.
When the standard hand circular saw is placed on the workpiece, first the swinging protective hood contracts the workpiece in a ski-like manner. After its pre-opening the saw teeth which project under the base plate contact the workpiece. Thereby the base plate cannot lie flush on the workpiece and the hand circular saw tilts. This cannot be brought to the working position for plunge saws without undesired contact and damage to the workpiece surface. Thereby a misuse of the standard hand circular saw formed as a plunge saw must be prevented.
The danger of springing of the saw during immersion in direct run takes place on the account that at the beginning the saw teeth are subjected first to very asymmetrical forces which under certain condition sum up in an impact-like manner. The forces sum up during occurring alternating hard zones for example knots results in alternation of the feed pace and leads to springing of the machine with possible fatal outcome for the operator or for people around him. This is especially dangerous when these acrobatic efforts, with the mounted gap wedge with the standard hand circular saw misuse occurs for example by placing the front edge of the base plate on the workpiece with simultaneous lifting of the swinging protective hood and as a result pressing the saw downwardly so that the saw blade immerses in the workpiece.
These considerations have been followed for the last 50 years for designing special hand circular saw constructions which are different from the standard had circular saws and to which are different from the standard hand circular saws and to which the plunge saws belong. They are used in saws with blind joints in hollow panels or wood roofs or in saws for openings in cooking working plates for adjusting of flushing basis. The plunge saws perform their functions satisfactorily. The following problems have been considered as unavoidable during the manipulation: unfavorable working positions of operators, convulsions of the hand in the intersecting angled holding position, springing of the machine despite the counter run saw.